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| United States Patent |
5,214,227
|
|
Zhou
, et al.
|
May 25, 1993
|
Low pressure dehydrogenation of light paraffins
Abstract
A process for the dehydrogenation of light alkanes which employs a
gallium/platinum catalyst on a magnesium/alumina spinel support. The
catalyst comprises 0.3 to 5 wt. % Ga and 0.1 to 5 wt. % Pt on a spinel
type support material characterized by the formula Mg.sub.x Al.sub.2
O.sub.3+x where x is a number from about 0.1 to 1.1. A water soluble Mg
salt is incorporated into this support prior to the impregnation of Pt and
Ga.
| Inventors:
|
Zhou; Ying (State College, PA);
Davis; Stephen M. (Baton Rouge, LA)
|
| Assignee:
|
Exxon Research & Engineering Company (Florham Park, NJ)
|
| Appl. No.:
|
811392 |
| Filed:
|
December 20, 1991 |
| Current U.S. Class: |
585/660 |
| Intern'l Class: |
C07C 005/333 |
| Field of Search: |
585/660
|
References Cited
U.S. Patent Documents
| 3972806 | Aug., 1976 | Antos | 585/660.
|
| 4056576 | Nov., 1977 | Gregory et al. | 585/660.
|
| 4080394 | Mar., 1978 | Antos | 585/660.
|
Primary Examiner: Pal; Asok
Attorney, Agent or Firm: Prater; Penny L., Naylor; Henry E.
Claims
What is claimed is:
1. A process for the dehydrogenation of light paraffins, said process
comprising contacting of said light paraffins under dehydrogenation
conditions with a catalyst consisting essentially of Pt and Ga on a Mg
aluminate spinel having the formula Mg.sub.x Al.sub.2 O.sub.3+x, where x
is a number from about 0.1 to 1.1.
2. The process of claim 1, wherein said catalyst contains a halide
component.
3. The process of claim 2, wherein said halide is chloride and wherein said
catalyst is sulfided.
4. The process of claim 1, wherein the Ga content is from about 0.3 to
about 5 wt. % and the Pt content is from about 0.1 to 5 wt. %, and the
spinel support material comprises Mg and Al.sub.2 O.sub.3 in a mole ratio
of from about 1 to 1.
5. The process of claim 4, wherein said catalyst comprises from about 0.5
to about 3 wt. % Ga and Pt is present in a range from about 0.2 to about
1.0 wt. % Pt.
6. The process of claim 4, wherein said catalyst contains a halide and has
been sulfided.
7. A process for the dehydrogenation of light paraffins, said process
comprising contacting said light paraffins under dehydrogenation
conditions with a catalyst consisting essentially of Ga and Pt supported
upon an magnesium aluminate spinel, wherein said catalyst is made in a
procedure comprising the steps:
(a) incorporating Mg into an Al.sub.2 O.sub.3 support material in a mole
ratio of Mg to Al.sub.2 O.sub.3 of from about 0.1 to 1.1;
(b) calcining said support material for an effective amount of time at an
effective temperature to form a magnesium aluminate spinel material;
(c) incorporating about 0.3 to about 5 wt. % Ga and 0.1 to 5 wt. % Pt into
said calcined spinel material.
8. The process of claim 7, employing a catalyst wherein Mg is incorporated
into said alumina support material prior to shaping the support.
9. The process of claim 1, which operated at a pressure of between 5 and 60
psia, a temperature between 450.degree. and 750.degree. C., a hydrogen to
oil ratio maintained at 1 or below, and a feed rate, gas hourly space
velocity of from 400 to 4000.
10. The process of claim 9, which is operated at a pressure between 10 and
30 psia, a temperature between 525.degree. and 625.degree. C., a hydrogen
to oil ratio maintained at 0.5 or below, and a feed rate, gas hourly
velocity of from 600-2000.
11. The process of claim 10 wherein said light paraffins comprise propane,
normal butane, isobutanes, pentanes and other saturated hydrocarbons from
the liquid petroleum gas range.
Description
FIELD OF THE INVENTION
This invention relates to a process for the dehydrogenation of light
alkanes which employs a gallium/platinum catalyst on a magnesium/alumina
support. Copending U.S. patent application Ser. No. 07/811,393, filed on
the same date as the present application, relates to the preparation of
this catalyst.
BACKGROUND OF THE INVENTION
The most frequently employed dehydrogenation reactions involve the
dehydrogenation of alkylcyclohexanes to aromatics; however, light alkane
dehydrogenation is increasingly being employed. The reason for this is the
growing enthusiasm for low emissions gasoline. The light alkane
dehydrogenation process normally involves conversion of propane, butanes,
or pentanes to the corresponding olefins, and the process configurations
are similar to those utilized in catalytic reforming. As compared to
catalytic reforming, the light alkane dehydrogenation processes typically
operate at higher temperatures and lower pressures and with more frequent
catalyst regeneration.
One of the best known methods for light alkane dehydrogenation is the
so-called oxidative dehydrogenation process. In this process the light
alkanes are reacted with oxygen over a suitably prepared mixed metal oxide
catalyst to produce a mixture of olefin, water, CO.sub.2, and unreacted
alkane. While high conversions combined with high olefin selectivities can
be achieved, this process has a number of disadvantages including loss of
fuel value due to water and CO.sub.2 formation and process operations that
are costly and difficult from the viewpoint of industrial hazards
associated with exothermic combustion reactions.
A more direct and preferred approach is direct dehydrogenation over a
suitable catalyst to produce olefins and molecular hydrogen. This
chemistry has recently received considerable interest, although high
reaction temperatures in the range of 500.degree.-650.degree. C. are
required to obtain a significant equilibrium yield (e.g., 15-50 wt. %) of
olefin. Moreover, under these reaction conditions, light
alkanehydrogenolysis to methane and ethane is a competing, undesirable
reaction. Most catalysts studied to date have not shown very high
selectivities for dehydrogenation versus hydrogenolysis or have suffered
from rapid catalyst deactivation necessitating frequent regeneration. As a
consequence, the process economics have not been clearly favorable. Large
incentives exist for catalysts which show improved resistance to
deactivation and that may be regenerated using simple procedures such as
air treatment.
Prior art catalysts for direct dehydrogenation of light paraffins are
mostly based on platinum on support materials such as silica, alumina,
modified aluminas, and zeolites. Frequently, alkali and/or alkali earth
oxide additives are included to improve stability and/or selectivity for
olefin production relative to methane and ethane. One family of prior art
dehydrogenation catalysts contain platinum and tin dispersed on an alumina
support modified to contain alkali and/or alkali earth metals. U.S. Pat.
No. 4,430,517, for example, discloses light paraffin dehydrogenation
catalysts comprising a platinum group component, a Group IVA component,
especially tin, an alkali or alkaline earth component, more than 0.2 wt. %
of a halogen component, and a porous carrier material, wherein the atomic
ratio of the alkali or alkaline earth component to the platinum group
component is at least 10. Preferably, the catalyst comprises about 1 to 3
wt. % potassium. The classic Houdry-type catalyst described in UK Patent
Application GB 2162082A employs chromium and potassium dispersed on
alumina. By contrast, European Patent Application 212,850 discloses light
paraffin dehydrogenation with catalysts containing a platinum group
component on a silicalite support which is substantially free of alkali or
alkali earth metals.
U.S. Pat. No. 4,547,618 discloses propane dehydrogenation catalysts
comprising ZSM-12 zeolite modified with platinum and magnesium or
manganese. Gallium has been noted as an important component in
dehydrocyclodimerization catalysts for selective conversion of C.sub.3 and
C.sub.4 alkanes to aromatics. U.S. Pat. No. 4,528,412 discloses a catalyst
employing gallium dispersed in moderate acidity, ZSM-5-type zeolites for
this purpose. PEP Review 85-3-3, "Aromatics from LPG," provided by SRI
International, also discusses uses for this catalyst. Catalysts for the
dehydrocyclodimerization process are also disclosed by A. H. P. Hall in
European Patent No. 162,636. U.S. Pat. No. 4,350,835 discloses the use of
Ga/H-ZSM-5 for ethane conversion to aromatics. Very recently, U.S. Pat.
No. 4,985,384 has disclosed gallium containing zeolite-Beta as a catalyst
for increasing aromatic yields during fluid catalytic cracking. Gallium
has also been noted as a component in light alkane dehydrogenation
catalysts. U.S. Pat. No. 4,056,576 discloses gallium oxide, gallium
sulfate, and gallium ions exchanged onto the surface of hydrated silica or
hydrated alumina, optionally modified with Pt, Pd, In, Cr, Tl, Ge, Sn, or
Zn. Selectivity for propane dehydrogenation to propylene over Ga.sub.2
O.sub.3 /SiO.sub.2 at 610.degree. C. was only 71.3%. British Patent No.
1,499,297 discloses dehydrogenation of C.sub.10 + paraffins over catalysts
containing platinum and gallium, indium, or thallium deposited on alumina
together with minor amounts of lithium or potassium. Gallium loadings of
0.2 to 1.0 wt. % are suitable, loadings below 0.5 wt. % are preferred.
Neither of these patents directly considers light paraffin dehydrogenation
over bimetallic PtGa catalysts or the use of supports such as MgAl.sub.2
O.sub.4 spinels. U.S. Pat. No. 4,902,849 discloses dehydrogenation of
C.sub.2 -C.sub.5 paraffins over catalysts comprising at least one
aluminate spinel selected from the group consisting of aluminates of Group
IIA metals and Group IIB metals, at least one metal selected from the
group consisting of nickel, ruthenium, rhodium, palladium, osmium,
iridium, and platinum, and at least one compound of a metal selected from
the group consisting of germanium, tin, and lead. This patent does not
consider the presence of Ga at all, nor is it drawn exclusively to
magnesium alumina spinels.
SUMMARY OF THE INVENTION
The present invention relates to a process for the dehydrogenation of light
paraffins, said process comprising the contacting of said light paraffins
with a catalyst comprising Pt and Ga on a spinel support comprised of Mg
and Al.sub.2 O.sub.3.
BRIEF DESCRIPTION OF THE DRAWING
The sole figure compares the percentage of propane converted to olefins by
various catalysts of the examples herein at specific exposure periods. The
effectiveness of a commercial preparation is compared to compositions
comprising Pt alone on a magnesium spinel support, Pt and Sn combined on a
magnesium spinel support, and Pt and Ga combined on a magnesium spinel
support.
DETAILED DESCRIPTION OF INVENTION
Aluminas suitable for use in accordance with the present invention are any
of the high purity aluminas suitable for use as a support for reforming
catalysts. The alumina can be synthetic or naturally occurring, although
synthetic alumina is preferred because its preparation can be controlled
to insure the appropriate level of purity and desired physical
characteristics. It is also preferred that the alumina be one which upon
calcination forms gamma alumina. By "an alumina which upon calcination
forms gamma alumina" it is meant an alumina which is essentially in the
trihydrate form prior to calcination, and which upon calcination is,
according to the crystal pattern, gamma alumina. Principally, these
aluminas are derived from precipitation methods or, preferably, the
digestion of metallic aluminum by a weak organic acid.
In a preferred precipitation method, the alumina is prepared by the
addition of an acid or acid salt such as hydrochloric acid or any of the
alums, to an alkali metal aluminate, such as sodium or potassium
aluminate.
The most preferred aluminas suitable for use herein are those prepared by
digesting, or reacting, metallic aluminum with a weak organic acid to form
an alumina sol. Preferred weak organic acids include acetic and formic
acid. It is also preferred that the aluminum be digested in the presence
of a mercury compound, such as a mercury aluminum hydroxide complex of
acetic acid. Such processes are well known to those skilled in the art and
are described in U.S. Pat. Nos. 2,274,634; Re 22,196 and 2,859,183; all of
which are incorporated herein by reference. As previously mentioned, in
such a process, an alpha aluminum salt is prepared by dissolving metallic
aluminum in a dilute (about 1-6 wt. %) organic acid in the presence of a
mercury compound. The aluminum and mercury form an amalgam which slowly
dissolves with the evolution of hydrogen to alumina salt containing
mercury, undissolved aluminum, and other materials. If desired, the
resulting sol can be treated with a sufficient amount of ammonium
hydroxide to obtain a pH of about 6.8 to 7.8, to form a gel which can be
dried and calcined. It is preferred that the sol not be gelled, but that
it be sprayed-dried to produce a high purity alumina hydrate powder, which
can then be ground to an appropriate particle size. Although not critical
for the practice of the present invention, an appropriate particle size is
from about 5 to 15 microns.
The magnesium component can be incorporated into the alumina during any
stage of the preparation of alumina as long as the mole ratio of Mg to
alumina is about 0.1 to 1.1. In a particularly preferred production scheme
for producing the alumina of this invention, high purity alumina hydrate
powder is first prepared by digesting metallic aluminum in a weak organic
acid, thereby forming an alumina sol which is then spray-dried by a
conventional spray-drying technique to produce the alumina hydrate powder.
If the alumina hydrate powder is not of appropriate particle size, it can
be ground by a conventional grinding means for reducing the particle size
of refractory powders. The alumina hydrate powder is then blended with an
effective amount of water, or sol, to form a paste of sufficient
consistency for extrusion.
Magnesium can be introduced into the alumina paste using a water soluble
magnesium compound such as magnesium nitrate, magnesium acetate, etc. or
as a finely divided hydrous oxide derivative of magnesium oxide such as
"magnesium hydroxide" (Mg(OH).sub.2 .multidot.xH.sub.2 O). After thorough
mixing, the magnesium-containing alumina paste is then extruded into an
appropriate shape such as cylindrical or trilobal pellets, dried and
calcined for one to several hours at temperatures from about 400.degree.
C. to about 700.degree. C. Calcination is preferably conducted at
600.degree. C. to 700.degree. C. Magnesium containing alumina supports
produced in this manner preferably exhibit characteristic features in the
X-ray powder diffraction pattern indicating partial or complete conversion
of magnesium and aluminum to magnesia alumina spinel, Mg.sub.x Al.sub.2
O.sub.3+x, where x is a number from about 0.1 to 1.1, preferably about 1.
It is more preferred that the magnesium be incorporated by blending the
alumina sol with a magnesium component, in the form of a water soluble
salt, prior to spray drying. The magnesium component can also be mixed
with the alumina powder prior to grinding. Although the magnesium
component can concurrently be incorporated into the alumina hydrate
material after extrusion by conventional impregnation techniques, it is
preferred to introduce the magnesium component prior to extrusion to
ensure homogeneity of the magnesium throughout the alumina material.
Suitable alumina supports can also be produced by extruding and calcining
an alumina paste to form gamma alumina followed by impregnation of a
soluble magnesium salt with drying and calcination at about 500.degree. C.
to 700.degree. C. under conditions similar to those used to produce the
alumina. This process is effective for depositing low concentrations of
magnesium. However, multiple impregnations may be required to achieve, the
preferred magnesium loadings depending on the pore structure and pore
volume of the alumina used.
Another approach for producing suitable magnesium-alumina support materials
has been reported by Rennard et al. (Journal of Catalysis, Vol. 98, Pg.
235, 1986) which involves coprecipitation of aqueous aluminum and
magnesium nitrates at pH 10 using dilute NH.sub.4 OH followed by
filtration, drying at about 100.degree. C. for about 18 hours, and finally
air calcination at about 600.degree. C. for about 18 hours.
The light alkane dehydrogenation catalysts of this invention are prepared
by incorporating Pt and Ga, metals capable of providing a
hydrogenation-dehydrogenation function, onto the Mg.sub.x Al.sub.2
O.sub.3+x support. The Pt will be present on the catalyst in an amount
from about 0.1 to 5 wt. %, calculated on an elemental basis, of the final
catalyst composition. Preferably the catalyst contains from about 0.2 to
about 1.0 wt. % Pt. The Ga content of the catalyst may range from about
0.3 wt. % to about 5 wt. %, preferably from about 0.5 to about 3 wt. % Ga,
based on the total weight of the catalyst (dry basis). Gallium to platinum
atomic ratios of 5 to 20 are preferred.
The Pt and Ga can be incorporated into the alumina by techniques such as by
impregnation either before or after it has been pilled, pelleted, beaded
or extruded. If impregnation is used, the modified alumina, in a dry or
solvated state, is contacted or otherwise incorporated with a platinum and
gallium salt and thereby impregnated by the "incipient wetness" technique.
Platinum and gallium can be impregnated sequentially with intermediate
drying and calcination or simultaneously. Simultaneous impregnation is
preferred. The incipient wetness technique embodies absorption from a
dilute or concentrated solution, with subsequent filtration or evaporation
to effect the total uptake of the metallic components. The solution used
in impregnation can be a salt or acid solution having the respective
platinum and/or gallium compounds dissolved therein. Chloroplatinic acid
and gallium nitrate are convenient precursors for catalyst preparation,
although other water soluble platinum and gallium compounds such as
Pt(NH.sub.3).sub.4 (NO.sub.3).sub.2, Pt(acetylacetanate).sub.2, or gallium
halides, acetates, etc. can be used with similar effectiveness. The
impregnation treatment can be carried out under a wide range of
conditions, including ambient or elevated temperatures, and atmospheric or
superatmospheric pressures.
The catalyst may also contain a halide component which contributes to the
acid functionality of the catalyst. The halide may be fluoride, chloride,
iodide, bromide, or mixtures thereof. It is preferred that the halide be a
chloride. Generally, the amount of halide is such that the final catalyst
composition will contain from about 0.01 to about 3.5 wt. %, preferably
less than about 0.5 wt. %, of halogen calculated on an elemental basis.
The halogen can be introduced into the catalyst by any method at any time
of the catalyst preparation, for example, prior to, following or
simultaneously with the impregnation of the platinum. In the usual
operation, the halogen component is introduced simultaneously with the
incorporation of platinum. Halogen can also be incorporated by contacting
the modified alumina in a vapor phase, or liquid phase, with a halogen
compound such as hydrogen fluoride, hydrogen chloride, ammonium chloride,
or the like.
The catalyst, after impregnation of Pt and Ga, is dried by heating to a
temperature above about 27.degree. C. preferably between about 65.degree.
C. and 150.degree. C., in the presence of nitrogen or oxygen, or both, in
an air stream or under vacuum. The catalyst can then be calcined at a
temperature from about 300.degree. C. to 650.degree. C., preferably from
about 400.degree. C. and 600.degree. C., in the presence of nitrogen or
oxygen in an air stream, or in the presence of a mixture of oxygen and
inert gas. This calcination, or activation, is conducted for periods
ranging from about 1 to about 24 hours in either flowing or static gasses.
Optionally, reduction is performed by contact with flowing hydrogen at
temperatures ranging from about 175.degree. C. to about 600.degree. C. for
periods ranging from about 0.5 to about 24 hours at about 1 to 10 atm.
Moreover, the catalyst may optionally be sulfided by use of a blend of
H.sub.2 S/H.sub.2 at temperatures ranging from about 175.degree. C. to
about 500.degree. C. at about 1 to 10 atm for a time necessary to achieve
breakthrough, or until the desired sulfur level is reached. Post-sulfiding
stripping can be employed, if desired, at conditions similar to those for
reduction of the catalyst.
The alumina spinel materials of this invention are characterized as: (i)
having a Mg to Al.sub.2 O.sub.3 mole ratio of about 0.1 to 1.1; (ii) a
surface area greater than about 50 m.sup.2 /g, preferably from about 125
to 200 M.sup.2 /g; (iii) a bulk density from about 0.6 to 0.9 g/ml,
preferably from about 0.7 to 0.8 g/ml; (iv) an average pore volume from
about 0.3 to about 0.7 ml/g, preferably from about 0.4 to about 0.5 ml/g;
and (v) an average pore diameter from about 75 to 150 .ANG..
The feed, or charge stock can be selected from propane, normal butane,
isobutanes, pentanes and other LPG (liquid petroleum gas) range saturated
hydrocarbons. These hydrocarbons are extremely volatile. Propane boils
within the range of about -46.degree. C. to -38.degree. C. at atmospheric
pressure, and commercial butane boils at about 9.4.degree. C.
The runs are initiated by adjusting the hydrogen and feed rates, and the
temperature and pressure to operating conditions. The run is continued at
optimum conditions by adjustment of the major process variables, within
the ranges described below:
______________________________________
Major Operating
Typical Process
Preferred Process
Variables Conditions Conditions
______________________________________
Pressure, psia
5-60 10-30
Reactor Temp., .degree.C.
450-750 525-625
H.sub.2 /Hydrocarbon
0-1 0-0.5
Molar Feed Ratio
Feed Rate, GHSV*
400-4000 600-2000
______________________________________
*(gas hourly space velocity = volume of gas per volume of catalyst per
hour)
The instant invention is illustrated further by the following examples
which, however, are not to be taken as limiting in any respect. All parts
and percentages, unless expressly stated otherwise, are by weight.
EXAMPLES 1 and 2, and COMPARATIVE EXAMPLES A-E
A series of catalysts with comparable loadings of platinum, tin, indium,
gallium, copper, lanthanum, and palladium was prepared using incipient
wetness impregnation methods employing aqueous H.sub.2 PtCl.sub.6,
SnCl.sub.2, In(NO.sub.3).sub.3, Ga(NO.sub.3).sub.3, Cu(NO.sub.3).sub.2,
La(NO.sub.3).sub.3, and Pd(NH.sub.3).sub.4 (NO.sub.3).sub.2. The support
materials used in these studies were a reforming high grade purity alumina
along with a magnesium alumina spinel (MgAl.sub.2 O.sub.4) that was
produced by coprecipitating aqueous aluminum and magnesium nitrate (in a
molar ratio of 2:1) at pH 10 at ambient temperature using NH.sub.4 OH.
This was followed by drying at 100.degree. C. for 18 hours, and calcinated
at 600.degree. C. for 18 hours. This series of catalysts was produced by
sequential impregnation of Pt or Pd followed by impregnation of Sn, In,
Ga, Cu or La as indicated. After each metals impregnation step, the
catalysts were dried in air, then in vacuum at 100.degree. C., and finally
calcined in air at 600.degree. C. for 18 hours.
The catalysts were evaluated for dehydrogenation activity in a small
downflow microreactor using a 1.5 g charge of nominally 14/35 mesh
catalyst and a feed gas blend of propane/hydrogen in a 3.3/1 molar ratio.
Standard conditions for the reaction studies were 605.degree. C., 1 atm
total pressure, and 65 cc/minute gas feed rate (e.g., ca. 2000 GHSV (gas
hourly space velocity=volumes of gas per volume of catalyst per hour), 2
second contact time). Activation was accomplished by hydrogen reduction
for 1 hour at 500.degree. C. followed by heating in flowing hydrogen to
600.degree. C. prior to introducing propane.
Table I below summarizes catalytic data obtained after 40-100 minutes on
feed for the above catalysts. Propane conversion and propylene selectivity
have been used as primary indicators of performance. Propylene selectivity
represents the fraction (percentage) of reacting propane molecules which
produce propylene.
Several features should be noted from Table I. For example, 0.6 wt. % Pt on
MgAl.sub.2 O.sub.4 showed respectable performance characterized by
moderate activity and selectivity, whereas palladium only showed poor
activity. A catalyst containing 2.5 wt. % Ga on MgAl.sub.2 O.sub.4 also
showed significant intrinsic activity for dehydrogenation along with very
high 95% selectivity. However, The combination of 0.6 wt. % platinum with
2.5-5.0 wt. % gallium produced exceptional catalysts with very high
activity and dehydrogenation selectivities. The improved selectivity
achieved relative to catalysts based on the individual components clearly
appears to suggest a synergistic interaction between platinum and gallium.
No performance credits were realized with a high gallium loading indicating
that the optimum gallium/platinum atomic ratio is somewhat less than 20.
Combining 0.6 wt. % platinum with other additives such as copper or
lanthanum produced changes in activity and selectivity, although none of
these catalysts displayed performance approaching that of the
platinum-gallium systems. It is also notable that the PdGa/MgAl.sub.2
O.sub.4 catalyst displayed inferior performance relative to
PtGa/MgAl.sub.2 O.sub.4.
TABLE I
__________________________________________________________________________
Catalytic Behavior of
Several Materials for Propane Dehydrogenation
Propane Propylene
Conversion.sup.(1)
Selectivity.sup.(1)
% at time on stream
% at time on stream
Examples
Catalyst 40 Min.
100 Min.
40 Min.
100 Min.
__________________________________________________________________________
Comp. A
0.6 Pt/MgAl.sub.2 O.sub.4
20 17 84 87
Comp. B
0.5 Pd/MgAl.sub.2 O.sub.4
3 3 71 72
Comp. C
2.5 Ga/MgAl.sub.2 O.sub.4
16 16 96 95
1 0.6 Pt-2.5 Ga/MgAl.sub.2 O.sub.4
33 31 98 98
2 0.6 Pt-5.0 Ga/MgAl.sub.2 O.sub.4
31 30 97 98
Comp. D
0.6 Pt-2.5 Cu/MgAl.sub.2 O.sub.4
20 18 93 94
Comp. E
0.6 Pt-2.5 La/MgAl.sub.2 O.sub.4
12 10 28 90
Equilibrium for conditions
37 100
__________________________________________________________________________
.sup.(1) @ 605.degree. C., C3/H2 = 3.3, 1 atm, 2000 GHSV
EXAMPLE 3 and COMPARATIVE EXAMPLES F-H
In order to better assess the behavior of catalysts containing platinum in
combination with elements from Group III and Group IV, a series of
catalysts was prepared containing 0.3 wt. % platinum and 1.0 wt. % indium,
tin, and gallium. As indicated in Table II below, addition of tin and
indium moderately improved the activity and selectivity of the base
platinum catalyst. However, none of these catalysts showed activity
approaching that of the platinum-gallium catalyst of the present
invention. Thus, it appears evident that the combination of platinum and
gallium produces catalysts with novel and special properties. It is
noteworthy that the PtGa catalyst is particularly superior to PtSn, since
the latter material is thought to be similar in terms of metals
composition to the platinum-tin catalysts employed in commercial light
alkane dehydrogenation technologies. It is also important to note by
comparison of reaction data collected at 10 minutes and 100 minutes, that
the PtGa catalyst displayed the lowest rate of deactivation among the
materials studied.
TABLE II
__________________________________________________________________________
Catalytic Behavior of Platinum in
Combination with Tin, Indium, and Gallium
Propane Propylene
Conversion.sup.(2)
Selectivity.sup.(2)
(% at time on stream)
(% at time on stream)
Examples
Catalyst 10 Min.
100 Min.
10 Min.
100 Min.
__________________________________________________________________________
Comp. F
0.3 Pt/MgAl.sub.2 O.sub.4
15 8 76 82
Comp. G
0.3 Pt-1.0 Sn/MgAl.sub.2 O.sub.4
21 11 91 90
Comp. H
0.3 Pt-1.0 In/MgAl.sub.2 O.sub.4
19 13 95 96
3 0.3 Pt-1.0 Ga/MgAl.sub.2 O.sub.4
30 27 95 96
__________________________________________________________________________
.sup.(2) @ 605.degree. C., C3/H2 = 3.3, 1 atm, 2000 GHSV
EXAMPLE 4 and COMPARATIVE EXAMPLES I-O
To gain further insight into the performance of PtGa/MgAl.sub.2 O.sub.4
relative to other materials, comparisons were carried out. One material
used was a catalyst comprising 0.3 wt. % Pt and 0.3 wt. % Re on Al.sub.2
O.sub.3 (Comp. Ex. I) prepared by loading with 3% potassium using
incipient wetness impregnation of KNO.sub.3, prior to calcination and
sulfiding. A second catalyst comprised 0.8 wt. % Pt on K-L zeolite (Comp.
Ex. J) and a third catalyst contained 2.7 wt. % chromium and 0.5 wt. %
potassium (Comp. Ex. M) dispersed on alumina. The latter material was
prepared to simulate the properties of the classic Houdry-type catalyst
described in U.K. Patent Application BG 2162082A. A commercial
CrK/Al.sub.2 O.sub.3 dehydrogenation catalyst containing about 4% chromium
was also evaluated. As indicated in Table III, all of these catalysts
exhibited initial propane conversion activity that was comparable to that
of PtGa-catalysts. However, none of these catalysts exhibited the high
dehydrogenation selectivities afforded by PtGa/MgAl.sub.2 O.sub.4
providing further evidence for the special and superior performance of
this system. Moreover, the PtGa/MgAl.sub.2 O.sub.4 catalysts showed
reduced deactivation rates relative to the other materials.
TABLE III
__________________________________________________________________________
Catalytic Behavior for Propane Dehydrogenation
Propane Propylene
Conversion.sup.(3)
Selectivity.sup.(3)
(% at time
(% at time
on stream)
on stream)
Example
Catalyst 40 Min.
100 Min.
40 Min.
100 Min.
__________________________________________________________________________
Comp. I
Sulfided 0.3 Pt-0.3 Re/Al.sub.2 O.sub.3
39 28 84 82
Comp. J
Sulfided 3 K/0.3 Pt-0.3 Re/Al.sub.2 O.sub.3
39 23 88 88
Comp. K
0.8 Pt/K-L zeolite
52 40 44 49
Comp. L
2.5 Cr/MgAl.sub.2 O.sub.4
24 21 95 94
Comp. M
2.7 Cr-0.5 K/Al.sub.2 O.sub.3
26 32 92 91
Comp. N
Commercial CrK/Al.sub.2 O.sub.3
43 78 27 90
Comp. O
2.5 Ga-2.7 Cr-0.5 K/Al.sub.2 O.sub.3
37 34 92 91
4 0.3 Pt-1.0 Ga/MgAl.sub.2 O.sub.4
30 27 95 96
5 0.6 Pt-2.5 Ga/MgAl.sub.2 O.sub.4
33 31 98 98
__________________________________________________________________________
.sup.(3) @ 605.degree. C., 1 atm, 2000 GHSV, C.sub.3 /H.sub.3 = 3.3
EXAMPLE 5
As noted above, the PtGa/MgAl.sub.2 O.sub.4 catalysts consistently
exhibited superior activity maintenance as compared to the other materials
investigated. This is shown more clearly in the Figure, which compares
propane conversion as a function of reaction time at 605.degree. C. for
five catalysts with different compositions.
EXAMPLE 6
Studies were also conducted using isobutane as a light alkane feedstock at
575.degree. C., 1 atm, GHSV=2400, and with a molar C.sub.4 H.sub.10
/H.sub.2 feed ratio of 3.0. Very stable activity and dehydrogenation
selectivity were observed with the 0.6% Pt-2.5% Ga/MgAl.sub.2 O.sub.4
catalyst over a period of 22 hours. At the end of this period, the
catalyst was subjected to a simulated air regeneration test by treatment
with air at 500.degree. C. and with GHSV=1500 for 2 hours. Subsequently,
the isobutane reaction was restarted. Table IV compares performance data
for isobutane dehydrogenation in the first and second reaction cycles. The
data indicate that PtGa/MgAl.sub.2 O.sub.4 is a robust catalyst that can
withstand a high temperature air treatment aimed at removal of coke
deposits. Moreover, these data clearly show that PtGa/MgAl.sub.2 O.sub.4
is a very effective catalyst for butane dehydrogenation.
TABLE IV
______________________________________
Isobutane Dehydrogenation Results
Isobutane Isobutene
Time on Feed Conversion.sup.(4)
Selectivity.sup.(4)
Cycle (hours) (mole %) (mole %)
______________________________________
1 1 41 95
1 21 41 96
-- air treat
2 1 43 95
2 6 37 97
______________________________________
.sup.(4) @ 575.degree. C., 240 GHSV, 1 atm
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